There has been a long-standing need for the capability of transmitting information rapidly and coherently from one location to another. Typically, the transmission of such information is performed either by way of a radio broadcast signal or by providing a signal through a conduit such as a wire.
Initially, signal transmission and reception was performed entirely with analog equipment. For example, a carrier radio signal of a specific frequency can be used to transmit voice, music and video signal information by amplitude and/or frequency modulation. By tuning to the specific frequency with a receiver, an interested party can receive the information.
Although effective, analog signals can and do suffer from a number of technical shortcomings. It is becoming increasingly more common to transform traditional analog information into digital form prior to or commensurate with transmission. Represented as a series of data bits such as logical “0” and “1” it is possible to achieve both high fidelity and perfect reproduction, depending of course on the choice of sampling rate.
Transmission of information in digital form is easily accomplished through the use of pulses. By adopting a convention, the presence of a pulse may be taken to be a logical “1”, the absence of the pulse being a logical “0”. Although the radio wave carrying the signal may be a traditional analog carrier wave, it is the pulse modulation in the wave that is being sent and received which corresponds to the digital information.
In an ideal setting with minimal noise, a comparator with a fixed threshold could be used to determine the presence and absence of each pulse in a detected signal. However, the real world is often far from ideal. Typically, an automatic gain control (ACG) device is used in connection with an RF receiver to collect the pulse signal. The AGC measures the signal energy in the receiver's passband, and adjusts the gain (within the constraints of an RC time constant) to normalize it to the receiver's output voltage range. In the case of an amplitude-modulated pulse signal, if the pulse signal is the dominant signal in the passband, the gain of the AGC will change with the pulse amplitude, keeping the amplitude of the detected pulses normalized to the receiver's output voltage range.
Unfortunately, the noise component in the signal comes from other sources and tends to remain constant as pulse amplitudes change. Since the noise amplitude is also altered by the AGC gain change, the effect is to change the output signal-to-noise ratio (S/N). This moves the ideal conversion threshold to various positions in the output voltage range, making a fixed conversion threshold impractical. Even worse, if no pulse signal is present, or if it fades away, noise becomes the dominant signal and the AGC will increase the gain until full-amplitude noise is produced. This condition can overload back-end processing with false pulses. Additionally, co-channel interference can appear as noise in the output, also making the S/N ratio variable. All of these phenomena can frustrate the recovery of patterns/information from the pulses in the received signal.
One partial solution to the aforementioned problems has been to provide an intermediate step designed to separate the pulse signal from the noise. This step is performed as an analog process, typically using a low pass filter/rectifier that is intended to filter out the pulses and return an integrated form of the noise, i.e. an integrated voltage level. This integrated voltage level is offset and used as a threshold by an analog comparator that compares the integrated output with the original signal to provide a digital pulse stream. Although such a process does work, there are several significant drawbacks.
For example, situations exist where the presence or absence of a signal may not be known a priori at a specific frequency. In these cases, noise-only signals may be input to the processing system. Subsequent processing of noise-only signals can and will result in false detections and can flood back-end signal processing circuitry with erroneous information.
Further, if the strength of the desired signal varies, the ability of the analog system to adjust is not very robust. The signal may be lost intermittently and not recognized for processing, or once again erroneous noise may flood the system.
Hence, there is a need for a method of signal-driven recovery of a digital pulse stream that overcomes one or more of the technical problems as stated above.